As is well known, a fuel cell is a system for generating electricity by directly converting oxygen and hydrogen contained in hydrocarbon materials such as methanol, ethanol, and natural gas into electric energy.
Fuel cells may be classified into different types based on the types of electrolytes used. Examples include phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells, and alkaline fuel cells. Although these fuel cells are basically operated in accordance with the same principles, these fuel cells are different from each other in terms of types of fuels, operating temperatures, catalysts, and electrolytes.
Among these fuel cells, recently developed polymer electrolyte membrane fuel cells (hereinafter, referred to as PEMFCs) have shown excellent output characteristics, low operating temperatures, and fast starting and response characteristics. Therefore, PEMFCs have a wide range of applications including as mobile power sources for vehicles, as distributed power sources for homes or buildings, and as small-sized power sources for electronic apparatuses.
A fuel cell system employing a PEMFC generally includes a stack, a reformer, a fuel tank, and a fuel pump. The stack is a main body of the fuel cell. The fuel pump supplies a fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate a hydrogen gas and supplies the hydrogen gas to the stack. As a result, the stack of the PEMFC generates electric energy through an electrochemical reaction between the hydrogen and oxygen.
The reformer in the fuel cell system generates the hydrogen from the hydrogen-containing fuel through a chemical catalytic reaction using thermal energy.
Generally, the reformer includes a reforming reaction section for generating the thermal energy to produce a hydrogen-containing reforming fuel from the fuel and a carbon-monoxide removing section for reducing the concentration of carbon monoxide contained in the hydrogen gas.
The reforming reaction section of the reformer is operated with exothermic and endothermic reactions using a catalyst. The reforming reaction section comprises a heat generating section for generating heat through a catalytic oxidation of a portion of the fuel reacting with air, and a heat absorbing section for absorbing the reaction heat and generating the hydrogen through a catalytic reforming reaction.
Since a conventional reformer comprises individual heat generating and absorbing sections, the heat generated by the heat generating section must be transferred to the heat absorbing section. Therefore, the heat generating and absorbing sections cannot directly exchange heat, so that the resulting heat transfer efficiency is low.
In addition, since a conventional reformer comprises individual heat generating and absorbing sections, it is difficult to implement a small-sized fuel cell system.
In addition, since the fuel must be heated during start-up before it is supplied to the reformer of a conventional fuel cell system, the system's performance efficiency deteriorates due to the energy consumption associated with the preliminary heating of the fuel.